Re: [PATCH RFC Dovetail 6.16 2/2] genirq: irq_pipeline: Migrate mask_lock to a hybrid spinlock

[Philippe Gerum <rpm@xenomai.org>]

Florian Bezdeka <florian.bezdeka@siemens.com> writes:

> The reason for the migration is the lockdep report below.
>
> The irq_desc_lock_class is the dovetail specific hybrid spinlock.
> pci_config_lock and mask_lock are both raw spinlocks.
>
> The behaviour of the pci_config_lock is different on kernels with
> IRQ pipelining enabled. While on plain Linux the critical section
> is entered with hard IRQs disabled, the inband stage is stalled with
> hard IRQs enabled on CONFIG_IRQ_PIPELINE. This is the reason why the
> pipeline entry code may still run on the same CPU and grab a
> (hybrid) IRQ descriptor lock.
>
> The critical section protected by the mask_lock was always entered
> with hard IRQs on.
>
> To break the cycle we migrate the mask_lock to a hybrid spinlock.
> This lock is only grabbed for IRQ setup. It's expected to have the
> lowest RT impact.
>
> irq_setup_affinity() is expected to be called from the inband stage
> only, so we will now enter the critical section with hard IRQs
> disabled. With that the pipeline entry code is locked out, breaking
> the cyclic lock dependency.
>

LGTM.

-- 
Philippe.